Fluid rotary joint and method of using the same

ABSTRACT

A fluid rotary joint has a stator with a generally curved stator body and a flex spline with a flexible annular band disposed about and secured to the stator body. The stator also has at least three radially extendable members disposed between the stator body and the annular band to deform the annular band away from the stator body to contact the inner surface of a rotor. The inner circumference of the rotor is greater than the outer circumference of the annular band. A driver selectively expands the extendable members and brings the annular band of the stator into frictional driving engagement with the rotor for rotating the rotor. The extendable members may also be selectively extended to allow the stator and rotor to freely move with respect to one another or to have limited contact with one another to act as a torque limiting device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/612,578, filed on Jun. 2, 2017, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/345,425,filed Jun. 3, 2016, the contents of which are incorporated in theirentireties herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract No.N00014-15-P-1130 awarded by the Office of Naval Research. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a strain wave actuator and, more particularly,to a fluid rotary joint using a strain wave actuator with an annularband instead of gears typically associated with a harmonic drive.

Description of Related Art

Directing attention to FIG. 1, a harmonic motor drive 10 typicallyutilizes three main components to reduce the speed of the input rotationof a wave generator and increase torque. Typical operation has acircular spline 20 fixed to the motor stator 30, a wave generator 40attached to the output of the motor rotor 50, and the flex spline 70 asthe output of a gearbox. The flex spline 70 has an oval shape where themajor axis A of the oval is rotated by the wave generator 40. It iscommonly stated in harmonic drive trade literature that the differencein the number of teeth 22,72 between the outside 74 of the flex spline70 and the inner surface 24 of the circular spline 20 generates themotion of the output. The flex spline 70 and circular spline 20 may havethe same gear pitch and different circumferences so that the flex spline70 must have fewer teeth 72 to mesh with the teeth 22 of the circularspline 20. However, more fundamentally, it is the difference in thecircumference of the flex spline 70 relative to the circular spline 20that produces the gear reducer effect whether gear teeth are present ornot. The gear ratio is a function of the difference in the circumferenceof the two gears 22, 72 and is entirely independent of the tooth sizesince the number of teeth in each gear is directly related to theirpitch diameters. The teeth, therefore, could be made of infinitesimalsize, or in fact there may be no teeth at all, with merely frictionalcontact engagement. The gear ratio will not be affected in the least byany such change in construction. The number of complete strain waverevolutions around the strain gear for one revolution of the outputelement is equal to the difference in pitch diameter of the drivenelement. This may also be presented in the following equation: GearRatio=(Ø Circular Spline)/(Ø Circular Spline-527 Flex Spline). Atraditional harmonic gearbox uses a mechanical gear wave generator todeform the flex spline into the circular spline.

SUMMARY OF THE INVENTION

In one embodiment, a fluid rotary joint has a stator with a generallycurved stator body and a flex spline with a flexible annular banddisposed about and secured to the stator body. The annular band has anouter surface with an outer circumference and a high tensile strength inthe direction of the circumference. The stator also has at least threeradially extendable members disposed between the stator body and theannular band to deform the annular band away from the stator body. Therotary joint also has a generally cylindrical rotor surrounding thestator body, wherein the rotor has a wall with an inner surface havingan inner circumference. The outer circumference of the annular band isless than the inner circumference of the rotor. A driver selectivelyexpands the extendable members and brings the outer surface of theannular band of the stator into frictional driving engagement with theinner surface of the rotor for rotating the rotor.

Another embodiment is directed to a method for using the fluid rotaryjoint comprising the steps of:

-   -   a) expanding at least one of the at least three radially        extendable members at a pressure greater than the remaining        members to deform the band against the inner surface of the        rotor thereby providing friction between the band and the inner        surface of the rotor; and    -   b) increasing pressure on an adjacent extendable member and        relieving the pressure on the at least one extendable member to        create relative motion between the annular band and the rotor.

These and other features and characteristics of a fluidic roll joint, aswell as the methods of operation and functions of the related elementsof structures and the combination of parts and economies of manufacture,will become more apparent upon consideration of the followingdescription and the appended claims with reference to the accompanyingdrawings, all of which form a part of this specification, wherein likereference numerals designate corresponding parts in the various figures.It is to be expressly understood, however, that the drawings are for thepurpose of illustration and description only, and are not intended as adefinition of the limits of the disclosure. As used in the specificationand the claims, the singular form of “a”, “an”, and “the” include pluralreferents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art sketch showing the operation of a harmonic drivemotor utilizing gears;

FIG. 2 shows the sequence (a)-(h) of inflations and deflationsgenerating a clockwise wave pattern to create continuous rotation of therotor in the opposite direction;

FIG. 2A and 2B are schematics depicting a fluidic harmonic actuator cellsequencing, from one pair of extendable members to two pairs beinginflated, causing the rotor to turn slightly;

FIG. 3 is a cross-sectional view of a fluidic rotary joint according tothe present disclosure;

FIG. 4 is a perspective view of a complete view of the cross-sectionillustration in FIG. 3;

FIG. 5 is a perspective view of the fluidic rotary joint with aschematic of the associated hardware according to the presentdisclosure;

FIG. 6 is a perspective view of the fluidic rotary joint with apneumatic drive ring assembly and circular spline removed;

FIG. 7 is a perspective view of the circular spline of the fluidicrotary joint with no friction treatment on the surface;

FIGS. 8A, 8B and 8C show one extendable member in perspective, in topview and in cross-section, respectively;

FIG. 9 is a perspective view of a pneumatic drive ring with diaphragmsinstalled;

FIGS. 10A and 10B are perspective and side views of the annular bandattached to the stator;

FIG. 11 is a perspective view of the stator body;

FIG. 12 is a side view of the annular band mounted within the statorbody;

FIG. 13 is a schematic of the fluidic rotary joint with associatedelements to operate the fluidic rotary joint; and

FIG. 14 is an expanded schematic of an overall system architecture planaccording to the present disclosure.

DESCRIPTION OF THE DISCLOSURE

For purposes of the description hereinafter, the terms “upper’, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”,“longitudinal”, and derivatives thereof, shall relate to the inventionas it is oriented in the figures. However, it is to be understood thatthe invention may assume alternative variations and step sequences,except where expressly specified to the contrary. It is also to beunderstood that the specific systems and processes illustrated in theattached drawings, and described in the following specification, aresimply exemplary aspects of the disclosure. Hence, specific dimensionsand other physical characteristics related to the aspects disclosedherein are not to be considered as limiting.

The present disclosure addresses the development of a lightweightfluidic rotary joint capable of continuous rotation.

The present disclosure provides a fluidic harmonic rotary joint capableof continuous motion, predictable small-step angles, and significanttorque production. The demonstrated joint has a 0.22° step size enablingprecise orientation control, and is capable of producing over 8 ft-lbsof torque. The design utilizes low-cost pneumatic actuators and aninexpensive friction material to transfer torque across the joint ratherthan the more typical, expensive, toothed surface. The low-cost designcombined with the fluidic drive mechanism make this an improvement forproducing roll motion for pneumatic or hydraulic robotic systems.

A fluidic rotary joint is a significant development for fluidicactuators because it has the ability to rotate with small precisionangle steps continuously in either direction or hold static torqueindefinitely in any orientation while energized only by a relatively lowpressure fluidic source. Furthermore, the motion generated isrotation-only, eliminating the axial motion-coupling characteristic ofsome other fluidic torsional actuators.

Extensive analysis and experience has demonstrated the need for atorsional actuator at the end of a manipulator. The prior artacknowledges the need for torsional actuator capability formanipulators, but also concedes that many current actuators have notbeen torque tested, have limited ranges of motion, and couple axialmotion with rotational motion. In one aspect, the fluidic rotary jointprovided in the present disclosure is an actuator that may be made oflightweight materials and driven with non-proportional on-off valves forcontinuous rotation in either direction. In one aspect, the joint itselfmay not be inflatable, but rather, it may add continuous torquecapability to an otherwise inflatable system. The concept for actuationis rooted in established strain wave gear reducer principles. However,there are two major points of departure for the fluidic rotary joint ofthe present disclosure that inspired the need to build hardware anddemonstrate that the flex spline may be driven by multiple discretepneumatic chambers instead of an oval shaped continuous wave generatorand torque may be transmitted through friction instead of gear teeth.

The fluidic rotary joint of the present disclosure is a modification ofthe more typical motor driven harmonic drive that utilizes a toothedinterface between the flex spline and the circular spline. The presentdisclosure takes advantage of the most fundamental principle of strainwave gearing to eliminate an expensive toothed interface and replace itwith a friction/shear torque transfer interface. Along with removing thegear teeth, the present disclosure implements a simple fluidicrotational drive to remove the need for a motor at the joint. Thehardware implementation of these improvements reduces cost, weight, andcomplexity.

The operation utilizes two adjacent sets of opposing diaphragms in orderto press a rubber material attached to the flex spine into the circularspline. A design to actuate the harmonic drive was developed utilizingCOTS pneumatic diaphragms to deform the flex spline as a departure fromthe traditional oval shaped wave generator. To generate rotationalmotion, one set out of two of the opposing diaphragms will remainpressurized as a pivot point while the pressurized adjacent set isvented and, simultaneously, the vented adjacent set is pressurized. Thesequence is repeated to produce rotary motion in either the clockwise orcounterclockwise direction, as shown in FIGS. 2(a)-2(h). The joint willalso resist static torques as the pressure is held constant in the setof opposing diaphragms. Unlike a typical harmonic drive, there is onlyone moving part in this design of the present disclosure. The wavemotion required to generate the motion between the flex spline and thecircular spline is generated by the diaphragms, which are fixed relativeto the flex spline. With reference to FIG. 2, fluidic harmonic actuatorcell sequencing is shown.

Overall, FIG. 2A illustrates a fluidic rotary joint 100 with a stator110 having a generally curved stator body 114. The stator 110 also has aflexible annular band 120 disposed about the stator body 114. Theannular band 120 has an outer surface 124 with an outer circumference C1and a high tensile strength in the direction of the circumference C1.The stator 110 further has a plurality of radially extendable members130 disposed between the stator body 114 and the annular band 120 todeform the annular band 120 away from the stator body 114. FIG. 2Aillustrates eight radially extending members 130.

A generally cylindrical rotor 140 surrounds the stator body 114 and hasa wall 144 with an inner surface 146 having an inner circumference C2.The outer circumference C1 of the annular band 120 is less than theinner circumference C2 of the rotor 140. As illustrated in FIG. 2A,radially extendable members 130A and 130B are in an extended positionwhereby the annular band 120 contacts the inner surface 146 of the rotor140 at contact regions R1 and R2. As a result, a portion of the annularband 120 does not contact, or lightly contacts, the inner surface 146 ofthe rotor wall 144 in at least gap regions G1, G2. The gaps G1, G2 are afunction of the difference in the circumference C1 of the outer surface124 of the annular band 120 and C2 of the inner surface 146 of the rotor140. Simply stated, by applying pressure by using the radiallyextendable members 130 and retracting and extending them in a patternsuch as the sequences (a)-(h) illustrated in FIG. 2, there is relativerotational motion between the annular band 120 and the rotor 140.Therefore, with the annular band 120 rotationally secured to the stator110 and as the radially extending members 130 are retracted and extendedsequentially, as illustrated in sequences (a)-(h) of FIG. 2, the rotor140 travels in a counter-clockwise direction. A driver 160 (FIG. 2A) isconnected to each of the extendable members 130 to extend and retracteach extendable member 130, as necessary. As illustrated in FIG. 2B, inorder to minimize slippage between the annular band 120 and the stator110, during the transition of extending adjacent expandable members 130,in one embodiment, prior to releasing an extendable member 130A, anadjacent extendable member 130C is extended. This is also illustrated inFIG. 2(b). Through continuous sequencing in such a manner as illustratedin FIG. 2, portions of the entire circumference C1 of the annular band120 may be radially extended thereby providing continuouscounter-clockwise displacement of the rotor 140.

As illustrated in FIG. 2A, the stator body 114 may be cylindrical.

The ability of the deformation of the annular band 120 to drive therotor 140 is based upon friction between the outer surface 124 of theannular band 120 and the inner surface 146 of the rotor wall 140.Contact between the outer surface 124 of the annular band 120 and theinner surface 146 of the rotor 140 may have a coefficient or friction ofbetween 0.01 and 2.0. Furthermore, the outer surface 124 of the annularband 120 may be selected from one of metal, plastic, rubber, andcomposites thereof while the inner surface 146 of the rotor wall 144 maybe selected from one of metal, plastic, rubber, and composites thereof.In one embodiment, the outer surface 124 of the annular band 120 may bemade from a liquid crystal aromatic polyester fiber. The inner surface146 of the rotor wall 144 may be made of aluminum. Furthermore, theouter surface 124 of the annular band 120 may be textured to provide afriction surface.

Finally, the inner surface of the rotor wall 144 may have teeth (notshown) extending radially inwardly to engage the outer surface 124 ofthe annular band 120, which would not have teeth.

FIGS. 3-12 illustrate one embodiment of the subject invention. Thereference numbers used herein will be the same as those used to describethe elements of FIGS. 2A and 2B but will be incremented by 100.

A detailed model of the fluidic rotary joint is shown in FIGS. 3-5. Thisdesign consists of a fixed stator that may include twelve (12) fluidicdiaphragms, or radially extending members 230 mounted in retainer plates232 of a drive ring 233 inside the flex spline, which is an annular band220. The flex spline will then push against the circular spline thatmakes up the rotor 240 of the fluidic rotary joint 200. A frictionmaterial is fixed to the outside of the flex spline annular band 220 toprovide the shear contact area between the surfaces that will drive therotor 240. The pneumatic drive ring 233 is part of the rotor 240 and isfixed so that pneumatic air lines 234 (FIG. 5) associated with theextendable members 230 are stationary. The air lines 234 are pressurizedand depressurized by a controller 235. The circular spline of the rotor240 is the output of the gear box, enabling continuous rotation of theoutput of the drive.

FIG. 6 shows the thin wall flex spline 220 with 1.13″ wide elastomericmaterial (the friction material) bonded to the outer diameter of theflex spline adjacent to its open end. FIG. 7 shows the rotor 240.

The mechanical wave generator of the prior art has been replaced with avirtual wave generator by sequencing the inflation and deflation ofpneumatic actuators arranged around the outside of a fluidic drive ring233. To drive a harmonic drivetrain, the force from the extendablemembers 230 on the inside of the flex spline 220 must be sufficient todeform the spline 220 and generate a normal force on the drive interfacebetween the flex spline 220 and the circular spline of the rotor 240. Inone embodiment, fiber reinforced diaphragms may be used. Such adiaphragm is capable of supporting up to a 150 psi differential pressurewithout failure and will apply a force in the piston direction acrossthe area of the face of the diaphragm. FIGS. 8A-8C show the size of thediaphragm, the direction of actuation, and the effective area thatapplies force to the flex spline. The diaphragm, or expandable member230, is shown in the deflated, or collapsed, configuration. The centralregion 231 is urged upwardly (FIG. 8C) when the diaphragm 230 ispressurized such that contact is made with the annular band 220 (FIG. 6)to urge it radially outward against the inner surface 246 (FIG. 7) ofthe wall 244 of the rotor 240. Approximately 40 psi of pressure in thediaphragm, across two opposing diaphragms in the drive ring, willgenerate sufficient deflection in the flex spline to push out againstthe circular spline. Since the joint torque capability should increaselinearly with pressure, the highest practical operating pressure up to150 psi should be used.

The pneumatic drive ring 233 is designed to provide the tightestpackaging possible of diaphragms used for the drivetrain. The drive ringis a single continuous ring to efficiently react to the pneumaticactuator forces in the plane of the actuators. The retainer plates 232are assembled to the pneumatic ring 233 of the stator 210 and machinedto slightly less than the anticipated inner minor diameter of the flexspline 220. The precision contour supports the energized flex splineminor diameter for torque transmission. Also, maintaining a minimum gapbetween the drive ring outer diameter and the flex spline helps tominimize the portion of the diaphragm that is radially unsupported. FIG.9 shows the fabricated and assembled pneumatic drive ring 233 with thediaphragms 230 (extendable members) in the deflated configuration. Asmentioned, pneumatic lines 234 (FIG. 5) are used to inflate or deflatethe diaphragms 230 as needed. The drive ring 233 is directly attached tothe stator body 214 (FIG. 3) and is therefore considered to be a part ofthe stator 210.

Directing attention to FIGS. 10A and 10B, the annular ring 220 ismounted to the rotor 240. As seen in FIGS. 11 and 12, the expandablemembers 230 are mounted within pockets 216 of the stator body 214. Onlythe outline of the stator body 214 is seen in FIG. 12. However, themechanism by which the expandable members 230 urge the annular ring 220against the rotor 240 is apparent.

Current harmonic drives utilize toothed surfaces to generate the drivingtorque for the mechanism. To reduce cost and complexity, the presentdisclosure uses a friction material in the interface between the flexspline 220 and the circular spline of the rotor 240.

As applied to the embodiment illustrated in FIGS. 3-12, a generallycylindrical rotor 240 surrounds the stator body 214 and has a wall 244with an inner surface 246 having an inner circumference C2. The outercircumference C1 of the annular band 220 is less than the innercircumference C2 of the rotor 240. As illustrated in FIG. 2a , radiallyextendable members 230A and 230B are in an extended position whereby theannular band 220 contacts the inner surface 246 of the rotor 240 atcontact regions R1 and R2. By applying pressure by using the radiallyextendable members 130 and retracting and extending them in a patternsuch as the sequence illustrated in FIGS. 2a-2h , there is relativerotational motion between and annular band 220 and the rotor 240.Therefore, with the annular band 220 rotationally secured to the stator110 then as the radially extending members 130 are retracted andextended sequentially, the rotor 240 travels in a rotational direction.A driver (not shown) is connected to each of the extendable members 130to extend and retract each extendable member 130, as necessary. Throughcontinuous sequencing of the extendable members 230, portions of theentire circumference C1 of the annular band 220 may be extended therebyproviding continuous counter-clockwise displacement of the rotor 240.

While the extendable member discussed herein has been directed to adiaphragm nested within a retainer plate, it is possible for theextendable member to be an independently inflatable bladder not nestedwithin the retainer plate.

At least two other materials may also be utilized to provide frictionbetween the deformable annular band and the rotor: Neoprene Rubber, 40Adurometer and 3M Gripping Material, GM110, nylon backed.

FIG. 13 is the system for the operation of the fluidic rotary joint 200while FIG. 14 is an expansion of the details of FIG. 13. In particular,the overall system to provide the functionality of the fluidic harmonicdrive is shown in FIG. 14. Operation of the system requires a regulatedcompressed air source, a bay of pneumatic valves to energize or vent thepneumatic actuator chambers, a power source for the pneumatic valves,electrical switches to sequence the actuator states, wiring to thepneumatic valves, hoses from the air source to the valves and pneumaticactuator chambers, and the rotary joint of the present disclosure.

The material utilized in the system of the present disclosure may becoated with a liquid crystal aromatic polyester fiber. This material islightweight, strong, and flex-fold damage resistant. With the coating,it can be made watertight. A laminated, hermetically sealed version, maybe 0.015″ thick and weigh 0.7 lbs/square yard.

The conceptual design for a fluidic harmonic drivetrain presented in thedisclosure was adapted to available commercial off-the-shelf componentscombined with custom manufactured components. The objective goal of +/−90° of rotation was achieved with a design capable of continuous rollmotion and generating appreciable torque. The drivetrain is also capableof fine, deterministic motion, necessary for the precise positioning ofobjects called for in the program solicitation. The capabilityrequirements for the inflatable structure and pitch joints were analyzedbased on application requirements and the additional capability driversimposed on the system by the fluidic harmonic drivetrain. The pressuresrequired to resist buckling and provide more deterministic motion werecalculated.

The fluidic harmonic drivetrain was successfully driven and shown to becapable of both precision stepping motion and large-scale continuousrotation. The resolution of the drivetrain, based on the design with 12inflatable diaphragms distributed around the circumference of thedrivetrain, was measured to be 0.22° . The resulting virtual gear ratioof the drivetrain is 134:1, and is driven by the ratio of the number ofvirtual revolutions of the deformation wave generated by the diaphragmsto the output motion of the circular spline. The expected gear ratio isthe ratio of the diameter of the circular spline divided by thedifference in diameters of the circular and flex splines, as was shownin the section on Fluidic rotary joint Design:

${{Gear}\mspace{14mu} {Ratio}} = {\frac{5.620\mspace{14mu} {in}}{{5.620\mspace{14mu} {in}} - \left( {5.620 - {2*{.025}}} \right)} = 112.4}$

It can be seen that the gear ratio is sensitive to the radial gap thatwas nominally intended to be 0.025″ between the flex spline outerdiameter and the circular spline inner diameter. Back-solving for theactual gap based on the measured 134:1 gear ratio yields an as-built gapof 0.021″. The small 0.004″ discrepancy can be explained by variationsin actual part dimensions relative to the nominal values, and adhesivethickness variation between the neoprene rubber and the flex spline.

The harmonic drivetrain is also capable of resisting significant torque.The torque coupling features of the present disclosure were used todetermine static holding torques with two sets of pneumatic diaphragmspressurized.

This harmonic drivetrain, utilizing simple fluidic actuators andinexpensive torque transfer material, enables motion that has not beenpossible with existing fluidic actuators.

The invention is also directed to a method for using the fluid rotaryjoint comprising method for using the fluid rotary joint of claim 1comprising the steps of a) expanding at least one of the at least threeradially extendable members 130 at a pressure greater than the remainingmembers 130 to deform the annular band 120 against the inner surface 146of the rotor 140 thereby providing friction between the annular band 120and the inner surface 146 of the rotor 140; and b) increasing pressureon an adjacent extendable member 130 and relieving the pressure on theat least one extendable member 130 to create relative motion between theannular band 120 and the rotor 140.

By extending and releasing the extendable members 230 in a rotationalsequence, the annular band 120 may be advanced in a single directionalong the inner surface 146 of the rotor 140. Furthermore, it ispossible prior to releasing an extendable member 130 to extend anadjacent member 130 to prevent slippage between the annular band 120 andthe rotor 140. Additionally, two or more extendable members 130 may beextended simultaneously to provide greater contact area between theextendable members 130 and the inner surface 146 of the rotor 140.

Furthermore, it is possible to relieve pressure among all of the members130 such that friction between the annular band 120 and the rotor 140 isde minimus and the annular band 120 may move freely relative to therotor 140 to produce a freewheeling configuration between the stator 110and rotor 140.

Finally, the pressure of the annular members 130 may be controlled suchthat friction between the annular band 120 and the rotor 140 is variedsuch that the torque transmission between the stator 110 and the rotor140 may be controlled.

While several aspects of fluid rotary joint are shown in theaccompanying figures and described hereinabove in detail, other aspectswill be apparent to, and readily made by, those skilled in the artwithout departing from the scope and spirit of the disclosure. Forexample, it is to be understood that this disclosure contemplates that,to the extent possible, one or more features of any aspect can becombined with one or more features of any other aspect. Accordingly, theforegoing description is intended to be illustrative rather thanrestrictive.

The invention claimed is:
 1. A fluid rotary joint comprising: a) astator having: 1) a stator body, 2) an annular band disposed about andsecured to the stator body, and 3) a plurality of extendable membersdisposed between the stator body and the annular band; b) a rotorsurrounding the stator body; and c) a driver connected to each of theextendable members.
 2. The fluid rotary joint in accordance with claim1, wherein the extendable members are equally spaced around an outersurface of the stator.
 3. The fluid rotary joint in accordance withclaim 1, wherein there are at least eight extendable members.
 4. Thefluid rotary joint in accordance with claim 1, wherein contact betweenan outer surface of the annular band and an inner surface of the rotorhas a coefficient of friction between 0.01 and 2.0.
 5. The fluid rotaryjoint in accordance with claim 1, wherein an outer surface of theannular band is made from a liquid crystal aromatic polyester fiber andan inner surface of the rotor wall is made of aluminum.
 6. The fluidrotary joint in accordance with claim 1, wherein an outer surface of theannular band is textured to provide a friction surface.
 7. The fluidrotary joint in accordance with claim 1, wherein an inner surface of therotor has teeth extending radially inwardly to engage an outer surfaceof the annular band.
 8. The fluid rotary joint in accordance with claim1, wherein each extendable member is a flexible inflatable diaphragm. 9.The fluid rotary joint in accordance with claim 1, wherein eachextendable member is an inflatable bladder.
 10. The fluid rotary jointin accordance with claim 1, wherein the annular band is supported aroundthe circumference of the stator.
 11. The fluid rotary joint inaccordance with claim 1, wherein the annular band has an outer surfacewith an outer circumference, and wherein the rotor has a wall with aninner surface having an inner circumference greater than the outercircumference of the outer surface of the annular band.
 12. The fluidrotary joint in accordance with claim 11, wherein the annular band has ahigh tensile strength in the direction of the outer circumference. 13.The fluid rotary joint in accordance with claim 1, wherein the driver isfor selectively expanding the extendable members and bringing an outersurface of the annular band of the stator into frictional drivingengagement with the inner surface of the rotor for rotating the rotor.14. The fluid rotary joint in accordance with claim 1, wherein theplurality of extendable members are radially extendable members disposedbetween the stator body and the annular band in order to deform theannular band away from the stator body.
 15. A method for using the fluidrotary joint of claim 1 comprising the steps of: a) expanding at leastone of the extendable members at a pressure greater than the remainingmembers to deform the band against an inner surface of the rotor therebyproviding friction between the band and the inner surface of the rotor;and b) increasing pressure on an adjacent extendable member andrelieving the pressure on the at least one extendable member to createrelative motion between the annular band and the rotor.
 16. The methodin accordance with claim 15, wherein the extendable members are extendedand released in a rotational sequence such that the annular bandadvances in a single direction along the inner surface of the rotor. 17.The method in accordance with claim 16, wherein prior to releasing anextendable member, an adjacent extendable member is extended to preventslippage between the annular band and the rotor.
 18. The method inaccordance with claim 15, wherein two or more extendable members may beextended simultaneously to provide a greater contact area between theextendable members and the inner surface of the rotor.
 19. The methodaccording to claim 15, further including the step of relieving pressureamong all of the members such that friction between the band and therotor is de minimus and the band may move freely relative to the rotorto produce a freewheeling configuration between the stator and rotor.20. The method according to claim 15, further including the step ofcontrolling the pressure among all of the members such that frictionbetween the annular band and the rotor is varied such that the torquetransmission between the stator and the rotor may be controlled.